This invention relates to methods for making solid electrolyte capacitors, and more particularly to the preparation of solid electrolyte capacitor anodes having a uniform profile.
Solid electrolyte capacitors typically comprise a sintered anode pellet formed with a dielectric oxide on the surfaces of the sintered particles of the pellet and have a dry solid electrolyte positioned in and around the pores of the pellet. These anodes are incorporated in a suitable casing after the application of a conductive counterelectrode over the solid electrolyte.
The size and weight of the uniform cased capacitor assembly is often considerably larger than that of the bare pellet because the solid electrolyte covering the pellet is typically very non-uniform. The solid electrolyte is typically applied by submersion in a manganese salt followed by heating to pyrolyze and convert the salt to a solid manganese dioxide electrolyte. In order to build a sufficiently thick electrolyte coating, these steps of immersing and pyrolyzing are repeated many times, e.g. 10- 15 times. The resulting surface is rough and uneven, tending to grow especially large in the regions of the corners of the pellet.
One solution to this problem has been to abrade or cut and form a uniform surface to the electrolyte coating. Besides increasing the cost, the mechanical abrading or cutting step tends to damage the pellet, reduce the yields, and shorten the useful life of the capacitor.
In U.S. Pat. Nos. 3,241,008 issued Mar. 15, 1966 and 3,481,029 issued Dec. 2, 1969 there are described methods for applying thick coatings of manganese dioxide to a pellet. The pellet is immersed in a gel or a sol containing manganous nitrate and silica powder. The thick coating thus applied is subsequently pyrolyzed. This substantially reduces the number of immersion and heating steps. The reduction in the number of heating steps tends to improve the yield and quality of the capacitors, and the surfaces of the electrolyte coating is smooth, typically appearing under the microscope as a mat of shiny nodules.
It is also known to substitute manganese dioxide powder for the silica in the slurry mixture with manganous nitrate, to reduce the number of heating steps and to produce a similarly uniform and smooth electrolyte coating.
These smooth solid electrolyte coatings, however, have the disadvantage that subsequent conductive counterelectrode coatings do not adhere well and now for a different reason, capacitor yields and life expectancy are reduced. It has been proposed that further immersions into pure manganous nitrate followed by pyrolyzing will provide a rougher surface to cure this problem, but the added heating step or steps are undesirable and the counterelectrode bonding is not rendered entirely satisfactory by this expedient.
It is therefore an object of this invention to provide a uniform solid electrolyte coating for a capacitor pellet to which secure bonding of a counterelectrode is readily made.
It is a further object of this invention to provide a method for forming a tough uniform solid electrolyte coating having a rough surface which method employs a minimum of heating steps.